
<!DOCTYPE html>
<html>

<head lang="en">
  <meta charset="UTF-8">
  <title></title>
  <style type="text/css">
    * {
      margin: 0;
      padding: 0;
    }

    ul {
      list-style: none;
    }

    .wrap {
      width: 330px;
      height: 30px;
      margin: 100px auto 0;
      padding-left: 10px;
      background-image: url(img/bg.jpg);
    }

    .wrap li {
      background-image: url(img/libg.jpg);
    }

    .wrap>ul>li {
      float: left;
      margin-right: 10px;
      position: relative;
    }

    .wrap a {
      display: block;
      height: 30px;
      width: 100px;
      text-decoration: none;
      color: #000;
      line-height: 30px;
      text-align: center;
    }

    .wrap li ul {
      position: absolute;
      top: 30px;
      display: none;
    }
  </style>
</head>

<body>
  <div class="wrap">
    <ul id="menu">
      <li>
        <a href="javascript:void(0);">一级菜单1</a>
        <ul class="ul">
          <li>
            <a href="javascript:void(0);">二级菜单11</a>
          </li>
          <li>
            <a href="javascript:void(0);">二级菜单12</a>
          </li>
          <li>
            <a href="javascript:void(0);">二级菜单13</a>
          </li>
        </ul>
      </li>
      <li>
        <a href="javascript:void(0);">一级菜单2</a>
        <ul>
          <li>
            <a href="javascript:void(0);">二级菜单21</a>
          </li>
          <li>
            <a href="javascript:void(0);">二级菜单22</a>
          </li>
          <li>
            <a href="javascript:void(0);">二级菜单23</a>
          </li>
        </ul>
      </li>
      <li>
        <a href="javascript:void(0);">一级菜单3</a>
        <ul>
          <li>
            <a href="javascript:void(0);">二级菜单31</a>
          </li>
          <li>
            <a href="javascript:void(0);">二级菜单32</a>
          </li>
          <li>
            <a href="javascript:void(0);">二级菜单33</a>
          </li>
        </ul>
      </li>
    </ul>
  </div>
  <script src="lib/jquery-1.12.4.min.js"></script>
  <script>
    var $onMenu =$("#menu>li");
    var $tuMenu =$("#menu ul");
    // $onMenu.mouseenter(function () {
    //   $(this).siblings().slideDown()
    // });
    // $onMenu.mouseleave(function () {
    //   $(this).siblings().slideUp()
    // });
  
    var instate = true;
    $onMenu.mouseenter(function () {
      if ($(this).children().is(":animated")) {
        return;
      }
      instate = true;
      $(this).children().slideDown()
    });
    $onMenu.mouseleave(function () {
      if ($(this).children("ul").is(":animated") && !instate) {
        return;
      }
      if (instate) {
        $(this).children("ul").slideUp()
      }
      instate = false;
    });
  
  </script>
</body>

</html>